This invention relates to spiral antennas and more particularly to center-fed multi-arm spiral antennas that are configured for transmitting both right and left-hand circularly polarized electromagnetic fields over a broadband of frequencies and/or receiving electromagnetic radiation of any polarization sense over a similarly wide frequency range.
It is well known in the art that center-fed multi-arm spiral antennas can be utilized to produce circularly polarized electromagnetic radiation with center-fed spiral antennas that are wound in the counterclockwise direction exhibiting right-hand circular polarization and antennas that are wound in the clockwise direction exhibiting left-hand circular polarization. Further, it is known that a center-fed multi-arm spiral antenna having N-arms or elements is capable of N-1 independent modes of operation by suitably establishing the phase difference between the excitation currents. In this regard, a first mode of operation (of mode order M=1) is attained when the phase difference between adjacent arms of the antenna is 2.pi./N. The M=1 mode is commonly referred to as the sum (or .SIGMA.) mode and produces a single-lobed radiation pattern that exhibits maximum field strength along, and symmetric about, the antenna boresight axis. Higher order modes (i.e., M=2,3, . . . , N-1), often called the difference (or .DELTA.) modes, are obtained by feeding the antenna such that the phase difference between adjacent arms is 2.pi.M/N and produce a radiation pattern that exhibits a null along the antenna boresight axis and maximum field strength along a cone of revolution about the boresight axis. In this respect, as the mode number increases a larger cone angle is exhibited between the imaginary line of maximum field strength and the antenna boresight axis and a decrease in relative field strength is exhibited.
An additional known characteristic of spiral antennas is that the radiation is emitted from substantially annular regions of the antenna in which the currents flowing through the adjacent arms are substantially in phase with one another. Because of the phase difference between the excitation currents and the spacial separation of the feed points this means that the radius at which maximum radiation occurs decreases as a function of frequency and increases as a function of the mode number M. For example, in a planar, tightly wound logarithmic spiral antenna, maximum radiation is often considered to occur at a radial distance of approximately M .lambda./2 .pi. from the center of the antenna where .lambda. denotes the freespace wavelength of the radiated signal.
In some situations a spiral antenna is utilized for transmitting or receiving sum mode radiation of a single polarization sense. In such a case, either a 2-arm spiral antenna or a single-arm arrangement in which the antenna arm is spaced apart from a ground plane can be employed. When such an antenna is to operate over a substantial frequency range, each antenna arm is dimensioned to accommodate the lowest frequency of interest and since radiation efficiency of 100 percent is not achieved, a portion of each arm current is not radiated within the previously mentioned annular region of the antenna, but continue to flow outwardly. If the associated antenna arm is of sufficient length, these residual currents reach secondary radiation regions in which an inphase relationship is attained and additional sum mode radiation of the intended polarization sense is emitted. Since such secondary radiation zones are spatially separated from the intended radiation zone antenna pattern degradation may result. Additionally, if the equivalent electrical length of the antenna arm is less than that required to produce secondary radiation, the residual arm current will be reflected from the outer terminus of the antenna arm and flow inwardly toward the antenna feed point. As described relative to the converted mode antenna arranges discussed in the following paragraphs, these inwardly flowing currents can cause radiation of a signal of the opposite polarization sense. When the antenna is intended to produce sum mode radiation of a particular polarization sense, producing an oppositely polarized radiation component may be an undesirable characteristic.
In addition to situations that require the transmission or reception of sum mode radiation of a particular polarization sense, there are many situations in which it is desired or necessary to transmit both left-hand and right-hand polarized signals or receive signals regardless of the polarization of the incident electromagnetic energy. Accordingly, several attempts have been made to adapt spiral antennas for such operation. For example, it has been recognized that a spiral antenna that is wound in a particular direction (clockwise or counterclockwise) exhibits a specific sense of circular polarization (right-hand of left-hand) when center-fed and the opposite sense of polarization when fed from the outer ends of the antenna arms. In this regard, the feed current phase relationship that causes operation in the "ith" mode (i.e., M=i; i=1, 2, . . . ,(N-1) when center-fed at the inner terminals produces radiation of the opposite polarization sense in the (N-i) th mode (i.e., M=N-i; i=1, 2, . . . ,(N-1) when the feed currents are applied to the outer ends of the antenna arms.
Although spiral antennas which provide operation with both left-hand and right-hand circular polarization by utilizing the inner and outer terminations of each antenna arm as signal terminals are satisfactory in some situations, several disadvantages and drawbacks are encountered. The primary limitation is that such configurations can only operate over a relatively narrow bandwidth (i.e., less than an octave). Additionally, as compared to a center-fed arrangement, twice as many signal terminals are required and configuring the antenna so that the impedance of the outer feed points is substantially identical to the impedance exhibited by the centrally located feed points can present problems.
A second technique that has been employed to configure spiral antennas for operation with both right-hand and left-hand circular polarization utilizes a spiral antenna having only the inner (or, alternatively the outer) terminations of the antenna arms connected to the associated transmitting or receiving system wherein higher excitation modes are, in effect, converted to lower operating modes of the opposite polarization sense. Spiral antennas utilizing this technique are typified by the antenna structure disclosed in Kuo et al, U.S. Pat. No. 3,562,756 and Ingerson, U.S. Pat. No. 3,681,772.
In accordance with the teachings of the Kuo et al patent, a multi-arm, center-fed spiral antenna is configured such that the length of the antenna arms and hence the radius of the antenna is less than that required to emit radiation at one or more of the higher operating modes. Considering such an arrangement from the standpoint of a transmitting antenna, this means that when excitation currents that would normally produce a higher mode of radiation are applied to the antenna terminals, the currents travel outwardly through the antenna arms and are reflected from the terminations thereof to flow inwardly toward the center of the antenna. Thus, center-fed currents at the higher excitation modes are, in effect, converted to inwardly flowing currents which produce radiation having a sense of polarization opposite to the radiation produced by the outwardly flowing currents that are induced when the antenna is excited at one of the lower modes. Since little signal attenuation occurs and since the phase relationship of the reflected arm currents is identical to that necessary to produce a lower mode of operation with the opposite sense of polarization, a N-arm spiral antenna configured in this manner can supply (N-1)/2 modes of both polarization senses (right-hand and left-hand) when N is an odd integer and (N-2)/2 modes of both polarization senses when N is even. For example, one arrangement disclosed in the Kuo et al. patent utilizes a planar spiral antenna having six elements that are wound in the counterclockwise direction in a manner which would normally produce right-hand circularly polarized radiation at operating modes M=4 and M=5 within regions of the antenna having a circumference greater than 2.75 .lambda. (i.e., a radius greater than 1.375 .lambda./.pi.), where .lambda. is the freespace wavelength of the transmitted signal. To effect the discussed converted mode operation, the antenna arms are terminated so that the circumference of the antenna is 2.75 .lambda.. Thus, exciting the antenna so that the phase difference between adjacent arms is 5.pi./3 radians (300.degree. ) does not produce right-hand circularly polarized radiation at M=5 (the fourth difference mode) but produces left-hand circularly polarized radiation at M=1 (the sum mode). Similarly, center feeding the antenna so that the phase difference between adjacent arms is 4.pi./3 radians (240.degree. ) does not result in right-hand circularly polarized radiation in the M=4 mode, but results in left-hand circularly polarized radiation in the M=2 mode (the first difference mode). Thus, simultaneously or selectively supplying feed currents to the center terminals of this antenna which would normally produce right-hand circularly polarized radiation at M=1, M=2, M=4 and M=5, produces sum and difference modes (M=1 and M=2 modes) of both right-hand and left-hand circular polarization sense.
The primary disadvantage of achieving converted mode operation by terminating the antenna arms in the manner taught by the Kuo et al patent is that such antennas are only suitable for use over a relatively narrow frequency range. In this regard, the circumference of such an antenna must be equal to or greater than that required to emit radiation in the normal manner at the desired lower modes of operation when the antenna is excited at the lowest frequency of operation and must be less than or equal to the circumference at which radiation of the higher, converted operating modes would normally occur when the antenna is excited at the highest frequency of interest. Because of these conflicting constraints, even such an antenna that includes eight elements and is arranged to supply sum and first difference mode radiation with both left-hand and right-hand polarization is restricted to operation over a bandwidth of one octave or less.
The above-referenced patent to Ingerson discloses spiral antenna arrangements in which signal reflection and, hence, converted mode operation is attained by controlling the effective electrical length of each antenna arm rather than by physically terminating the antenna arms. In the disclosed arrangement, identified as a modulated arm width (MAW) spiral antenna, each antenna arm comprises a series of "cells" formed by a section of antenna arm having a first, relatively narrow width dimension followed by a section of antenna arm of substantially greater width dimension. These cells or "modulations" are positioned along the antenna arms to establish impedance discontinuities or reflection regions (denoted as "stopbands" in the Ingerson patent) which are intended to selectively reflect the outwardly flowing currents. In particular, since maximum signal reflection occurs when the length of a cell corresponds to .gamma./2, utilizing a plurality of modulations in each antenna arm with cell length increasing as a function of the distance between the center of the antenna and the location of a particular cell, in effect, causes each arm to exhibit an effective electrical length that is inversely proportional to the frequency of the excitation signal. Thus, by also establishing the position of the arm width modulations (cells) so that currents produced by selected higher modes of excitation are reflected whereas the lower modes produce radiation in the conventional manner, operation is achieved with both left-hand and right-hand circular polarization. As is the case with other center-fed spiral antennas that utilize converted mode techniques, currents that would normally establish radiation at one of the higher modes M=i establishes radiation of the opposite polarization sense at an operating mode M=N-i and at least 2M.sub.m +1 antenna arms are required to effect both right-hand and left-hand polarization at modes M=1, 2, . . . ,M.sub.m.
Although modulated arm width spiral antennas of the type disclosed in the Ingerson patent are operable over a frequency range that substantially exceeds the bandwidth of previously proposed converted mode spiral antennas (e.g., those disclosed in the Kuo et al patent), substantial problems and drawbacks are still encountered. In particular, the stopbands do not provide substantially total reflection of the excitation currents that are to be converted into lower mode radiation of the opposite polarization sense and a significant portion of the antenna current continues to flow outwardly through the antenna arms. When the circumference of such an antenna is established for operation over a substantial bandwidth, most of the currents that pass beyond the stopbands cause higher mode order radiation with a polarization sense opposite to that of the desired converted mode radiation. Since the currents intended to induce converted mode operation are not totally reflected, the relative field strength of each converted mode differs from that of the corresponding lower mode of operation in which no signal reflection is induced. Further, the undesired radiation at the higher order modes may cause asymmetry of the radiation patterns relative to the antenna boresight axis. Thus, the characteristics of a modulated arm width antenna are both frequency and polarization dependent. Moreover, modulated arm width antennas are subject to inherent geometric constraints that can make it difficult to attain the desired electrical characteristics. In this respect, the conductor width required to achieve the desired modulation may conflict with the desired wrap angle (curvature of the antenna arms) and the requirement that length of each cell be .pi./2 may not permit each antenna arm to include as many cells as are necessary to effect uniform performance relative to variations in frequency.
In many applications the above-discussed non-ideal performance of a modulated arm width spiral antenna either causes substantial compromises in system performance and/or requires utilization of relatively complex compensating circuits. For example, amplitude monopulse tracking systems or angle of arrival systems that are independent of received signal polarization and continuously operable over a multi-octave frequency band require an antenna having radiation patterns that are highly symmetric about the antenna axis and independent of both frequency and polarization sense. In this regard, such systems detect the angle of arrival of an incident signal by determining the ratio between the signal induced at the difference mode and the signal induced at the sum mode of like polarization sense. In particular, the amplitude of the detected ratio corresponds to a "cone angle" that defines a cone of revolution about the antenna boresight axis which contains a line between the antenna and the source of radiation and the relative phase angle of the ratio corresponds to a "clock angle" which indicates the element along the surface of the cone of revolution that corresponds to the line between the antenna and source of radiation. To maintain proper relationship between the magnitude of the difference mode/sum mode radio (.DELTA./.SIGMA.) and ensure that the phase of the ratio varies linearly with clock angle, the radiation pattern of each sum and difference mode must exhibit virtually complete symmetry about the antenna boresight axis. Further, to permit the accuracy of such a system to be independent of received signal polarization, the two sum mode radiation patterns and the two difference mode radiation patterns must be of identical geometry.
Accordingly, it is an object of this invention to provide a broadband spiral antenna which includes means for controlling and reflecting the induced arm currents in a manner that reduces or eliminates undesired radiation characteristics such as secondary radiation.
It is another object of this invention to provide an improved broadband spiral antenna that is configured for operation with both left-hand and right-hand and circularly polarized radiation fields.
It is yet another and related object of this invention to provide a N-arm center-fed spiral antenna in which excitation currents that would normally result in radiation at a selected set of mode orders (N-1), (N-2), . . . (N-M) undergo substantially total reflection to produce radiation corresponding to the opposite sense of polarization at mode orders 1, 2, . . . , M.
It is still another object of this invention to provide a multi-arm spiral antenna which exhibits substantially identical characteristics relative to radiation of both left-hand and right-hand circular polarization to thereby provide an antenna suitable for use with high accuracy angle of arrival and amplitude monopulse tracking systems.